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.Dd March 30, 2020
.Dt POSIX_FADVISE 2
.Os
.Sh NAME
.Nm posix_fadvise
.Nd give advice about use of file data
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In fcntl.h
.Ft int
.Fn posix_fadvise "int fd" "off_t offset" "off_t len" "int advice"
.Sh DESCRIPTION
The
.Fn posix_fadvise
system call
allows a process to describe to the system its data access behavior for an
open file descriptor
.Fa fd .
The advice covers the data starting at offset
.Fa offset
and continuing for
.Fa len
bytes.
If
.Fa len
is zero,
all data from
.Fa offset
to the end of the file is covered.
.Pp
The behavior is specified by the
.Fa advice
parameter and may be one of:
.Bl -tag -width POSIX_FADV_SEQUENTIAL
.It Dv POSIX_FADV_NORMAL
Tells the system to revert to the default data access behavior.
.It Dv POSIX_FADV_RANDOM
Is a hint that file data will be accessed randomly,
and prefetching is likely not advantageous.
.It Dv POSIX_FADV_SEQUENTIAL
Tells the system that file data will be accessed sequentially.
This currently does nothing as the default behavior uses heuristics to
detect sequential behavior.
.It Dv POSIX_FADV_WILLNEED
Tells the system that the specified data will be accessed in the near future.
The system may initiate an asynchronous read of the data if it is not already
present in memory.
.It Dv POSIX_FADV_DONTNEED
Tells the system that the specified data will not be accessed in the near
future.
The system may decrease the in-memory priority of clean data within the
specified range and future access to this data may require a read operation.
.It Dv POSIX_FADV_NOREUSE
Tells the system that the specified data will only be accessed once and
then not reused.
The system may decrease the in-memory priority of data once it has been
read or written.
Future access to this data may require a read operation.
.El
.Sh RETURN VALUES
If successful,
.Fn posix_fadvise
returns zero.
It returns an error on failure, without setting
.Va errno .
.Sh ERRORS
Possible failure conditions:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument is not a valid file descriptor.
.It Bq Er EINVAL
The
.Fa advice
argument is not valid.
.It Bq Er EINVAL
The
.Fa offset
or
.Fa len
arguments are negative,
or
.Fa offset
+
.Fa len
is greater than the maximum file size.
.It Bq Er ENODEV
The
.Fa fd
argument does not refer to a regular file.
.It Bq Er ESPIPE
The
.Fa fd
argument is associated with a pipe or FIFO.
.It Bq Er EIO
An I/O error occurred while reading from or writing to a file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.El
.Sh SEE ALSO
.Xr madvise 2
.Sh STANDARDS
The
.Fn posix_fadvise
interface conforms to
.St -p1003.1-2001 .
.Sh HISTORY
The
.Fn posix_fadvise
system call first appeared in
.Fx 9.1 .
